bitintlower: Fix .MUL_OVERFLOW overflow checking [PR114038]

Currently, bitint_large_huge::lower_mul_overflow uses cnt 1 only if
startlimb == endlimb and in that case doesn't use a loop and handles
everything in a special if:
      unsigned cnt;
      bool use_loop = false;
      if (startlimb == endlimb)
        cnt = 1;
      else if (startlimb + 1 == endlimb)
        cnt = 2;
      else if ((end % limb_prec) == 0)
        {
          cnt = 2;
          use_loop = true;
        }
      else
        {
          cnt = 3;
          use_loop = startlimb + 2 < endlimb;
        }
      if (cnt == 1)
        {
          ...
        }
      else
The loop handling for the loop exit condition wants to compare if the
incremented index is equal to endlimb, but that is correct only if
end is not divisible by limb_prec and there will be a straight line
check after the loop as well for the most significant limb.  The code
used endlimb + (cnt == 1) for that, but cnt == 1 is never true here,
because cnt is either 2 or 3, so the right check is (cnt == 2).

2024-02-22  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/114038
	* gimple-lower-bitint.cc (bitint_large_huge::lower_mul_overflow): Fix
	loop exit condition if end is divisible by limb_prec.

	* gcc.dg/torture/bitint-59.c: New test.

diff --git a/gcc/gimple-lower-bitint.cc b/gcc/gimple-lower-bitint.cc
index 13b9b205df79fd2949c9208b3303a4ae1f3f8e5c..fb03063f86e4b2f72e7c0c7ec00f671a6de74a60 100644 (file)
--- a/gcc/gimple-lower-bitint.cc
+++ b/gcc/gimple-lower-bitint.cc
@@ -4497,7 +4497,7 @@ bitint_large_huge::lower_mul_overflow (tree obj, gimple *stmt)
                                           size_one_node);
                  insert_before (g);
                  g = gimple_build_cond (NE_EXPR, idx_next,
-                                        size_int (endlimb + (cnt == 1)),
+                                        size_int (endlimb + (cnt == 2)),
                                         NULL_TREE, NULL_TREE);
                  insert_before (g);
                  edge true_edge, false_edge;

diff --git a/gcc/testsuite/gcc.dg/torture/bitint-59.c b/gcc/testsuite/gcc.dg/torture/bitint-59.c
new file mode 100644 (file)
index 0000000..ef17424
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/bitint-59.c
@@ -0,0 +1,22 @@
+/* PR tree-optimization/114038 */
+/* { dg-do run { target bitint } } */
+/* { dg-options "-std=c23 -pedantic-errors" } */
+/* { dg-skip-if "" { ! run_expensive_tests }  { "*" } { "-O0" "-O2" } } */
+/* { dg-skip-if "" { ! run_expensive_tests } { "-flto" } { "" } } */
+
+#if __BITINT_MAXWIDTH__ >= 129
+int
+foo (unsigned _BitInt(63) x, unsigned _BitInt(129) y)
+{
+  return __builtin_mul_overflow_p (y, x, 0);
+}
+#endif
+
+int
+main ()
+{
+#if __BITINT_MAXWIDTH__ >= 129
+  if (!foo (90, 0x80000000000000000000000000000000uwb))
+    __builtin_abort ();
+#endif
+}